Purification of zinc electrolytes



June 10, 1952 J. T. ELLSWORTH 2,599,816

PURIFICATION OF ZINC ELECTROLYTES Filed Sept. 22, 1948 Cal sine Replenishmenta Iron Acid 0911 Linunr and Additions Magnesia.

Water v 1 Acid oxidizing agent added after silica, etc. u u Neutralizin 5' agents conversion added after leaching is complete and temperature is 2 Thickener below 50 0.

I Overflow 3 '11 r mun! Eeafiue Filtrate G.Qll.6&.L l l I l l Residue Leach 8 Purification l 1 e a 222. r ,9 J Usee 3 Filter Cud y es e 10 u F "'1 I I i L !52 f J rurii'ied m Solm ion ----1 Dry Residue Leeched lectro vtic Ge la g 1:! 4:: m =3 6 i Depleted Acid Cell Liquor M etellic zinc IN VEN TOR.

JOHN T. ELLSWOR'IH BY Zeal aze/flaw ATTQ RNEYS Patented June 10, 1952 2,599,816 PURIFICATION OF ZINC ELECTROLYTES John T. Ellsworth, Collinsville, Ill.; Mildred Ellsworth administratrix of said John T; Ellsworth,

deceased Application September 22, 1948, Serial No. 50,625

12 Claims.

This invention relates to the preparation and purification of zinc-bearing electrolytes from which zinc is to be recovered by electrolysis, and particularly to the treatment of such electrolytes for the removal of soluble impurities which would have a deleterious effect.

In the usual process of electrolytically recovering zinc from zinciferous materials, the zinciferous material, such as a basic calcine of roasted zinc concentrate, is leached with a sulfuric acid solution to form a leach liquor which is neutralized and then purified with respect to iron, and with respect to copper and cadmium and other impurities which follow their reaction. The purified solution is then subjected to electrolysis during which the zinc is precipitated out of the solution and a corresponding amount of sulfuric acid is generated in the electrolytic cells. The cell liquor, depleted in zinc and containing the generated sulfuric acid, is returned to the leaching tanks for treatment of further amounts of the zinciferous material. Thus, the cell liquor or solvent is cyclic, in that nearly all of it is used over and over again for extracting zinc from the zinciferous material, with only such replacement of acid and water as are necessary to replace natural losses, and losses occurring as a result of reaction of the sulfuric acid with components of the zinciferous material other than the zinc compounds.

Iron present in the leach liquor is in the form of ferrous sulfate. This ordinarily is oxidized to ferric sulfate by air blowing, while the leach liquor is hot, or by the addition of an oxidizing agent such as potassium permanganate, so that when the solution is neutralized and made slightly basic, the iron present as an impurity will precipitate in a coagulated form as'ferric hydrate. As the coagulated ferric hydrate settles it carries along a limited amount of gelatinous silica which separates from the solution on neutralization. In this manner a considerable reduction of the stickiness due to the presence of the gelatinous silica, is also obtained.

The copper and cadmium impurities are removed by agitating the leach filtrate with metallic zinc dust which precipitates such. metallic impurities from the solution.

The zinc calcine or other zinciferous materials contain certain soluble compounds which are not removed with the iron, copper and cadmium and which tend to build up in the cyclic solution to concentrations that deleteriously efiect the preparation of the zinc-bearing electrolyte and the electrodeposition of zinc therefrom. For simplicity these compounds are herein referred to as harmful soluble salts, and while referred to may be present in amounts such that the leach liquor will contain up to 5grams per liter, and iron.

' When the'concentration of the harmful solu-.. ble salts in the solution approaches or reaches their saturation points, they may be in part dispersed throughout the solution in a colloidal or gelatinous state. This is particularly true with! respect to silica and alumina-which, onneu tralization of the leach liquor or pulp, precipitate and separate from it in a gelatinous or colloidal form, the alumina precipitating as aluminumhydroxide. The presence of such harmful solu-- ble salts in too high a concentrationreduces" the efficiencies of the normal chemical reactions, particularly the removal of copper and cadmium, the filtering of both the leach and purification residues, and the electrodepositionof the zinc. and purification residues is greatly retarded by Since the filtration of the leach the presence of 'the gelatinous or colloidal im-- purities, a large amount of stray zinc solution is retained in the rejected residue. Consequently, a fairly large percentage of zinc content of the leach liquor is lost at this point, since it is difiicult to separate efficiently by a water or acid wash.

In my application Serial No. 723,209, now abandoned, filed January 20, 1947, as a continuation-' in-part of my application Serial No. 431,055, filed February 16, 1942, now Patent No. 2,454,142, I

have disclosed and claimed a method of removing the harmful soluble salts from the cyclic solvent solution by the addition of an agent or agents, usually magnesia, which will combine with alumina and silica present in the leachliquor to form a granular, insoluble complex or compound which is easily removed by filtration,

or which causes a change in the colloidal precipitate such that it does not interfere with the filtering of the leach and purification residues.

As a result of further investigations, I have: found that the dissolving of the zinc from the zinciferous material and the removal of harmful soluble salts by the addition of suitable precipitation agents is more effective and complete if during the leaching the acid leach is not allowed to go below grams per liter of acid, and, agitation of the pulp of the cyclic solvent solution and the zinciferouslmaterialiis continuedlfori a...minimumof about.. two,hours.gwithout useeitherof the heat; of reaction between the acid and the zinciferous material or heat applied extraneously. The agie tation should be conducted for a substantial length of time after the temperaturerofithezpulu has dropped to below 50 C. and preferably-until... the temperature of the pulp liasrdi'oppeditol'alooutl. C. or 0., and until the colloidal silica and alumina is converted to a. readily-filterable, granular form. With such prolonged agitatiom. it is possible, with the addition of suitable;a.ddietionor precipitation agents, to remove iron as well..;as a lumina.and silica from .the leach liquor without loss, during the usual1thickening; filtering andlwashihg, operations; of j an appreciable amount of.'the. zinc-contained iin the. leach liquor. Tli'atjis...particularly, true..when the oxidation of ferrousiron is delayed. for. a. length of time sufii' cientltdallow mostoflit to. reactor .combine with silica andi 'added lime-to form a granular, inso1i1ble;.and. readily filterable complex or compoundbfiromlime, andsilica... v

L'eaches -01.zinciferous..material. attain .a. temperature.oflaboutfifiic; or. more as a result of theireaction. of the. acidl in. the cyclic..solution. with. the'zinciferonsmaterial; Ihsmall .leaches, the heat of' reaction may be dissipated'and the. pplpucooled to about 25? C.. withinabout. two

Larger. leaches...will.take three..-hours or... longer Thus, the-extraction.:o the; zincifer.ous.- materiallwiththe .cyclicsolvent{solution ,will take. place. forla. substantial .lengthof time.helow.50.". which.Lhaveioundto.bemost.important. Pref: erablw. the agitation, of; the... pulpshould. con: timiesforl at least.one.hour.beforel it is neutralized andaforl at leastatwoehours, and.preferably at leastethreeehours aftenit. has ,been neutralized and-.has, cooled to below 50 'C.., and always should. bee; continued; until. the. temperature ofthe. pulp hasadropped to be1ow C.., as;a-.readily.-filter.-- ableresidue .containinggthe..iromas well as other. impuritiesallresent,twill not belobtained until the. temperaturahasedroppedeto that -extentnarily,gthe.=-agitation is continued until .thetemperatureshas. dropped;to:l about 25 C..- Duringthez-agitationthe temperature .of .thelpulp is .per: mittedsto .d-rop naturally and-.noattempt-is; made. tmanplyiaheat: or to. holdxthe; temperature at any point r I At. temperatures-70f: about 50.?" C. the silica andalumina which are present will be in gelatinous state: southat the residues filter. with: great .difficultyj-andrentrapa substantial-proportion oi the- Atz. temperatures; below-about .50? C, the aluminasgand Sfli-CflqWfll'iShOlV- B; colloidal form :for: aasubstantialrperiod lofztimegz butwhen .the =agita--- tiomis continued foria sufflcientlengthof. time,- such;as:for:two.hoursor.more after. the-pulp has cooled;.-to below-.:50-.-- C.,,and in theapresence-of. suitable. additioneor precipitation. agents, a high percentageof.the'gelatinousrsilica and, alumina is; converted into: a: granular-. insoluble form which H Ordiethe leach liquor is neutralized by the addition of magnesia, or additional calcine. The neutralized leach liquor should contain sufucient calcium sulfate and magnesium sulfate to combine with the silica, alumina and iron present. As the leach liquor ordinarily will not contain sufiiicent magnesiumsulfateand calciumsuliatefor. that purpose; the leach liquor." ordinarilyi willlb'e neutralized with magnesia and lime. Lime which is added to neutralize the leach liquor, should not be added until the temperature of the leach liquon: droppedf. to; 50 C. and preferably not until .thetemperature has dropped to about C.. becauseeattemperatures of C. or higher, the zinc contentofltheeleach liquor will react with the lime to form a precipitate from which the zinc-"contentis noti readily recoverable, with resultantloss-of such zinc from the solution. However, at temperatures of about 45 C. or lower reaction between the lime-and the zinc contentoi the" leach: liquoris'negligible and practically; no zinc*will be'precipitated' from the solution'bythe lime and'therebylost.

The use" of both lime and magnesia for new tralizing the leachliquoris preferred as the'lime will combine with the silica and iron to form a" granular; insoluble compound or' complex of iron; lime and'silica'; whilethe magnesia will combine with the alumina and silica to'forma granular, insoluble compoundor 'complexof'alumina, magnesia and silica. If magnesia were -used exclusively for neutralizing the'aci'd of the leach liquor; not i only would the complex or compound containingttheiron notibe formed, but the solution usually wouldcontain'an excess of. the amount of magnesia? necessary" to;.-. formthe alumina-mag nesia-silica compoundzor complex; and the magnesia would build 'up'togan. undesirable amount" inpthe: cycle; with the" result that the: solution would be" too basic :and'som'e' zinc-wouldbe'pre cipitatedi during 'the conversion step; either as hydroxide: or some; indeterminate basic" complex;

As" thersurfaceportionof thelime particles react-withthe-acidiofthe leach' liquor to" form insoluble coatingspf'calcium sulfate; and-as lime tends to lump'or'b'ail up; the .lime prefera'blyis' added" in fih61yT"diVidd'if0rm intermittently in small amounts:" At"first"only sufiicientflimeris added to neutralize-theacid: Thereafter; ad ditions are made to.replace the calciumsulfate which is withdrawxrfrom' the'solutionby reaction or. combination with'the iron orsilica to form'- the granular, insoluble iron-limeesilica' compound '2 or complex'and' to' provide an excess of about 1 gramper.litentoalways insure that lime or calcium sulfate is 'presenttfor reaction or. combination with the iron andsilicato form "a granular; insoluble iron-lime-silica-compound or complex.

Lime, when added to theleach'liquor at a temperature of 45C. orlower, is a particularly good neutralizing agent-as the calcium sulfate formed during-the neutralization of the: acid. entersIthe-i system and is:available-for'reaction'or combination with the. iron'and silica to;form a-granular;

' insoluble compound or complex Gypsum ..crystalsalso may be. added to the leach liquor for. this purpose.

As the magnesia .do.es-not so readily react with thezinc content ..of."..the...said.leach liquor, itmay. be. added. thereto, before. theetemperaturel has. dropped .toibelow 502 Gland preferably, is. added. in such amount astomaintainacontent of from 2.to 4 gramsper liter in thesolution. This, along with the maintaining. of a slight excess-of. lime-.is. a simple and easy way to maintain the proper basicity of the leach liquor. It also insures the presence of magnesia for the formation of the alumina-magnesia-silica compound or complex. Excess basicity must be avoided, because if the solution becomes too basic zinc compounds will form which are insoluble, and their zinc content will be lost.

The presence of the magnesia is necessary to react or combine with the alumina and silica to form the granular, insoluble magnesia-aluminasilica complex. As that compound or complex forms more readily, and as the iron-line-silica compound or complex does not form substantially until the solution is made more basic, I prefer to first add sufiicient magnesia to combine or react with the alumina and silica to form the granular, insoluble alumina-magnesia-silica compound or complex, and later to add the lime to complete the neutralization and to form the granular, insoluble iron-lime-silica compound or complex. If desired, the magnesia may be added during the leaching of the calcine or other zinciferous ma terial, but the lime should preferably not be added until the temperature of the leach liquor has dropped to below 50 C. The addition of the magnesia and lime can be made in the same leaching tank, successively, or the leach liquor may be filtered from the residue remaining from the leaching operation, and the lime added to the leach filtrate. However, I prefer to add the magnesia and lime successively in the leach tank.

Calcine is not effective to form either the alumina-magnesia-silica or the iron-lime-silica compound or complex, and when added to the leach liquor performs merely a neutralizing function. j

After the neutralization of the leach liquor begins, the pulp should be agitated for at least two hours, and preferably for three hours or longer. During this period no heat is added and no attempt is made to retain the heat of reaction between the acid and zinciferous material, the temperature being permitted to drop naturally and gradually until it is below 30 C., and preferably is down to 25 C. or lower, if convenient.

As soon as the leach liquor is neutralized, some of the silica will begin to separate in a gelatinous form. However, if sufficient lime and magnesia are present, most of it will combine with lime and iron, and with the magnesia and alumina, to form granular, insoluble compounds or complexes, and thereby be readily removable from the system by filtration.

' As the reaction or combination of the silica and the calcium sulfate takes place with the iron while it is in the ferrous state to form the iron-limesilica compound or complex, the oxidation of the ferrous sulfate of the solution is postponed until after the formation of the iron-lime-silica and the alumina-magnesia-silica compounds or complexes have been completed. However, as it is necessary to completely precipitate the iron from the solution because of its deleterious effect during the purification of the leach liquor with respect to the copper and cadmium, and during the electrolytic operation, and also because arsenic and antimony are simultaneously removed, the oxidation of the ferrous iron should take place as soon as the reaction or combination of the ferrous iron with the lime and silica seems to be complete, and before metallic zinc dust is added to the leach liquor to separate copper and cadmium therefrom.

The invention will be further described in connection with the accompanying flow sheet which is illustrative of one method of carrying out the invention.

Zinciferous material in the form of a roasted zinc concentrate, or calcine, is added to a bate tery of leaching tanks I and suificient depleted electrolyte or acid cell liquor added to react with the zinc content of the calcine, and to leave an excess of about 1% of acid. The pulp of calcine and acid cell liquor in each leach tank is agitated for about one and one-half hours, when substantially all the zinc content of the calcine will have been dissolved by or reacted with the acid of the cell liquor, and the acid content of the cell liquor will have been reduced to slightly above 1%.

Calcines will vary with respect to the amounts of nonzinciferous material which they contain. If the particular calcine being treated does not contain sufficient iron to form the granular, insoluble iron-lime-silica compound or complex referred to above, and leave a slight excess for oxidation to ferric hydrate, sufficient ferrous iron for that purpose, usually in the form of ferrous sulfate, is added during the leaching. However,

most calcines will contain ample iron and the addition of ferrous sulfate ordinarily is not necessary. During the leaching of the calcine, I also add sufiicient magnesia to maintain a magnesium sulfate content in the leaching liquor of about 3 or 4%, calculated as MgO, and add sufficient water and acid to make up for that lost during the cycle.

After the leaching of the calcine has been completed and the temperature of the leach liquor has dropped to about 45 0., sufficient lime in finely divided form is gradually added to neutralize the remaining acid and, thereafter to replenish that which combines with the iron and silica to form the iron-lime-silica compound or complex, and the agitation of the pulp continued for about three hours, or until the formation of the granular, insoluble compounds or complexes of iron, lime and silica, and alumina, magnesia and silica is complete, at which time the temperature of the leach liquor will have dropped to between about 20 C. and 25 C. Sufiicient oxidizing agent, such as potassium permanganate, is then added to each leach tank to oxidize the remaining ferrous sulfate to ferric sulfate, which precipitates in coagulated form, as ferric hydrate, and, as it settles, carries along most of the gelatinous silica which did not combine to form the insoluble complexes with the iron and lime, and with the alumina and magnesia.

A small amount of calcine preferably is added to neutralize any acid generated as a result of the oxidation of the ferrous sulfate.

The pulp is then passed to a thickener 2 and filter 3, where it is decanted, thickened and filtered in accordance with the usual procedure. The filtering removes the leach residue along with the granular, insoluble complexes of iron, lime and silica, and alumina, magnesia and silica, as well as the coagulated ferric hydrate and gelatinous silica carried along with it, and all other matter not soluble in the filtrate.

The filtrate is then passed to a battery of tanks 4, where it is agitated with metallic zinc dust for precipitation of copper and cadmium. It is then filtered in a filter 5 for the removal of the preclpitated copper-cadmium residue, and the purified filtrate then passed to the electrolytic cells 6, where deposition of the zinc content of the electrolyte takes place. After the electrolysis has proceeded to the stage where the electrolyte is 73* soidepleted iwitlnzmcithatselectrolysismozlongeri is efiicient, the depleted electrolyte is withdrawn" andzpassedibacle'to the leaching tanks. I for: the leaehing'of .fu'rther amounts of calciner Thilesidll .zfromithe filter.'3 may, if fdesiredg. be' 'conveyedtoa tankt :where it is leached with cell .acid, or water, for 'the recovery ofzthe' iron whichtwas precipitatedaszferric hydrate, aswell as 'for-therecovery ofany zinc which precipitates. asc-zinc :hydratei The leach residuexis separated; fr'omitheleach liquor in the filter 9 and the leach liquorsor filtrate added-to the .acid cell liquor be-- ing returned to .the leach tanks. Asmostof the: sili'cazwas rendered insoluble by combination-with iron and lime, and with the alumina and;mag;-= nesiaathe smallamount returned to the cycle with thee-leach liquor or filtrate'from the'filter 9-will no-tbe'excessive.

If .desired, the improvements in the .purification of the zinc electrolyte maybe combined with thecmethod vof purifyingsuch electrolytes disclosed and'claimed in my aforesaid patent, wherepurification" is obtained by withdrawing and evaporatingto dryness a small amount of the electrolyte and then leaching the dry residue with ancequal amount of water to pick up the zinc, which is soluble, whileleaving the harmful salts which are rendered'insoluble by the evaporation of-:the residue. When that method of purification'iscombined' with the present method, up to 10% of the purified electrolyte leaving the filter 5 is passed to an evaporator H) where it is evaporated to dryness, after which the dry residue is'leached in a leach tank I I with an amount of water equal tothat which Was'evaporated from the electrolyte-in'the evaporator ID, for the re covery of its zinc content. The leach liquor from the-:leaching' tank ll containing, the recovered zinc iseadded to tthe'electrolyteprior to its-introduction .into the electrolytic cells .6.-

It will be understood that where referencehas beenmade herein, and is made in the appended claims -to the solution or to the granular, insoluble-complexes'containing magnesia or "lime; those compounds are 'given-asreported by analyses of theecomplexes. While themagnesium and cal? cium must be present in the solution in the form oftheir. respective sulfates, the actual chemical formula of the complexes is unknown; likewise with respect to the iron, alumina, etc.

The prolonged agitation of the pulp of zinciferous material and the leaching liquor with the magnesiaand lime at temperatures below 50 0., together with the postponement-of the oxidation OfEfEII'OUSfiIOIl' which doesrnot react or combine withithelime and silica toform the iron-limesilica compound or complex; until afterthe conversion :of the silica and-alumina to a granular,

' readily-filterable' form and until after-the formationoftthe'iiion-lime-silicacomplex and the complex: of: alumina; magnesia and silica :have been completed; results. in. a much better. recovery of theacidesoluble zinc content tof the: zinciferous. material and'the .productionof a zinc electrolyte which contains much lessioftheharmfulsoluble salts.

Irclaim'z l. IntheLmethod of recovering zinc fromtzmciferous' materialstinvolvingithe leaching of the zinciferous .materials .with :a cyclic .sulfuric. acid so'lutionttoxform a zinc-containing-solution which subsequently is :subjected toelectrolysis for the electrodeposition of zinc therefrom; the improvement Iin reducing, .prior'to electrodeposition, the concentrationv in the solution of soluble salts which-exert 1azharmfulzeffectizduringz'electrblysis whenipresentinioohighasconcentratiomzwhich: comprises neutralizin'gsthe.solutionsbyetheaiddis tion theretomf: limeand magnesia i'insadditionrto'.

: anylimepr m'agnesia originally containedimthe;

zinciferousymaterial, the :lime :being. added :to the; solution while the solution is at a temperature. below 50. C.. but notsubstantially below"t20.?C., and agitating the solution after: the addition-pf;

lime 'andrwh'ile. it is at said temperature for a'tpeie.

riod 'of-iat "least two 'hours andJuntil iron-and: at least the major portioniof silica and. alumina presf-v ent in'ithe .solution .in .gelatinousiform: areecone? vertedito readily=filterable, granular-.complexesiofi. iron;.lime andsi'li'ca; and alumina, magnesia-and. silica...

2'. In the method. ofrecovr-ing .zincfromfzinc iferous materials "involving the. leaching :jof 'th'e: zinciferousmaterials :with a cyclicssulfu'ric acid. solution to form 'a zinci-containing'solution which subsequently is subjected to electrolysis for thi electrodeposition of zincthere'from; the improve; ment :in :reducin'g; prior to electrodepositionjithe concentration in the solution: of "soluble salts which exert'a harmful effect during electrolysis when present in too high a'concentration,"which" comprises neutralizing the-solution by the 'addi-' tion thereto of lime and magnesia'in additiorrtoi any limeor magnesia originally contained in the? zinciferous material, the limeb'eingaddedto the. solutionwhile-the solution is at a temperature: above about 20 C. but not above about 45 CI, and agitating the "solution after: the addition of .1 limeand while it istat saidtemp'eraturefor aiperiod of at least two :hours :anduntil ironxand at least the major portion of the silica and alumina pres ent in-Ltl'ie solutionini gelatinous form. are. converted into readily=filterable; granular complexes? of TlI'OIl, lime "andsilica; and :alumina, magnesia and silica:

3; Inv the methodiofrecoveringzinctfrom'zinc iferous materials: involving the. leaching offthe zinciferous materials with a. cyclic: sulfuric acid' solution'to form a zinc-containing solution which. subsequently" "is .subjectedito electrolysis fori'the' electrodeposition of zinc therefrom; the. improveme'nt inreducing; prior-to: electrodeposition; the concentration inthe SOIUtiOIlTOf solublesalts': which exert'a harmfullelfectiduring electrolysis when present in'too high .a-"concentration; whichcomprises neutralizing the "solution by the I addi tion thereto of lime and imagnesia in 1additionfto' any lime or magnesia originally contained irr-thezinciferousmaterial, the 'lime being, addedtltoithe solution while the solution? is at a temperature" below 59 C. butnot substantially below 20 C;,' agitating the solution'afterthe addition of lime and whileit .isat said temperature for a :period of atleasttwo hours and until iron-and at least themajor .portion of the silica andalumina present in thesolution in gelatinous form are convertedinto readily-filterable, granular complexes of iron lime :and. silica, and alumina; .magnesia and silica; and thereafter/adding an oxidizing agent to the :solution to. oxidize: ferrous iron :remaining therein to the-ferric;state.-

4. In the method ofrecovering-zinc-from zinm iferous materials involving the leaching of. the. zinciferous.materials'with acyclic sulfuric acid soluticnto form a zinc-containing solution which subsequently is. subjected.- to. electrolysis for; the. electrodeposition of zinc therefrom; the: improve-- ment in reducing, prior to electrodeposition, the concentration in the solution of' soluble salts which exert a. harmful efiect -during:electrolysis when present in too high a concentration, which comprises neutralizing the solution by the addition thereto of lime and magnesia in addition to any lime or magnesia originally contained in the zinciferous material, the lime being addedto the solution while the solution is at a temperature above about 20 C. but not above about 45 C., agitating the solution after the addition of lime and while it is at said temperautre for a period of at least two hours and until iron and at least the major portion of the silica and alumina that is present in the solution in gelatinous form are converted into readilyfilterable, granular complexes of iron, lime and silica, and alumina, magnesia and silica, and thereafter adding-an oxidizing agent to the solution to oxidize ferrous iron remaining therein to the ferric state.

5. In the method of recovering zinc from zinciferous materials involving the leaching of the zinciferous materials with a cyclic sulfuric acid solution to form a zinc-containing solution which subsequently is subjected to electrolysis for the electrodeposition of zinc therefrom; the improvement in reducing, prior to electrodeposition, the concentration in the solution of soluble salts which exert a harmful effect during electrolysis when present in too high a concentration, which comprises neutralizing the solution by the addition thereto of lime and magnesia in addition to any lime or magnesia originally contained in the zinciferous material, the lime being added to the solution at a temperature above 30 C. but not above about 45 C., agitating the solution after the addition of lime and while it is at said temperature for a period of at least three hours and until the temperature of the solution has dropped to below 30 C. and until iron and at least the major portion of the silicaand alumina present in the solution in gelatinous form are converted into readily-filterable, granular complexes of iron,

lime, and silica, and alumina, magnesia and silica, I

and thereafter adding an oxidizing agent to the solution to oxidize ferrous iron remaining therethe temperature has dropped to below 50 C. but

not substantially below 20 C., adding lime to complete the neutralization and to provide sufflcient lime for combining with iron and silica present to form a granular, insoluble complex of iron, lime andsilica.

7. In the method of recovering zincfrom zinciferous materials involving the leaching of the zinciferous materials with a cyclic sulfuric acid solution to form a zinc-containing solution which subsequently is subjected to electrolysis for the electrodeposition of zinc therefrom; the improvement in. reducing, prior to electrodeposition, the concentration in the solution of soluble salts which exert a harmful effect during electrolysis when present in too high a concentration, which comprises first adding magnesia to combine with alumina and silica present to form a granular, insoluble alumina-magnesia-silica complex, then, and after the temperature has dropped to at least 45 C. but not substantially below 20 0., adding lime to complete the neutralization and to pro vide sufiicient lime for combination with iron and silica present in the solution to form a granular, insoluble complex of iron, lime and silica.

8. The improvement in the method of recovering zinc from zinciferous materials set forth in claim 1, in which at the end of the leaching of the zinciferous material the leaching solution contains at least ten grams per liter of acid.

9. In the method of recovering zinc from zinciferous materials involving the leaching of the zinciferous materials with a cyclic sulfuric acid solution to form a zinc-containing solution which subsequently is subjected to electrolysis for the electrodeposition of zinc therefrom; the improvement in reducing, prior to electrodeposition, the concentration in the solution of soluble salts which exert a harmful effect during electrolysis when present in too high a concentration, which comprises neutralizing the solution by the addition thereto of basic material including magnesia in addition to any magnesia originally contained in the zinciferous material, and thereafter agitating the solution while it is at a temperature below 50 C. but is not substantially above 20 C. for a period of at least two hours and until at least the major portion of the silica and the alumina present in the solution in gelatinous form are converted into a readily-filterable, granular complex of alumina, magnesia and silica.

10. The improvement in the method of recovering zinc from zinciferous materials set forth in claim 9, in which theagitation is begun while the temperature of the solution is above 30 C. and is continued for a period of at least three hours and until the temperature of the solution has dropped to below 30 C.

11. The improvement in the method of recovering zinc from zinciferous materials set forth in claim 9, in which an oxidizing agent is thereafter added to the solution to oxidize the ferrous iron remaining therein to the ferric state.

12. The improvement in the method of recovering zinc from zinciferous materials set forth in claim 9, in which the agitation is begun while the temperature of the'solution is above 30 C. and is continued for a period of at least three hours and until the temperature thereof has dropped to below 30 C., and in which thereafter an odixizing agent is added to the solution to oxidize ferrous iron remaining therein to the ferric state.

JOHN T. ELLSWORTH.

REFERENCES CITED The following references are of record in the file of this patent: I

UNITED STATES PATENTS Number Name Date 1,363,052 Peterson Dec. 21, 1920 1,843,006 Steven et al. Jan. 26, 1932 FOREIGN PATENTS Number Country Date 361,258 Germany Oct. 12, 1922 592,354 Germany Feb. '7, 1934 OTHER REFERENCES Ralston, Electrolytic Deposition and Hydrometallurgy of Zinc, pages 40-73. 

1. IN THE METHOD OF RECOVERING ZINC FROM ZINCIFEROUS MATERIALS INVOLVING THE LEACHING OF THE ZINCIFEROUS MATERIALS WITH A CYCLIC SUFLURIC ACID SOLUTION TO FORM A ZINC-CONTAINING SOLUTION WHICH SUBSEQUENTLY IS SUBJECTED TO ELECTROLYSIS FOR THE ELECTRODEPOSITION OF ZINC THEREFROM; THE IMPROVEMENT IN REDUCING, PRIOR TO ELECTRODEPOSITION, THE CONCENTRATION IN THE SOLUTION OF SOLUBLE SALTS WHICH EXERT A HARMFUL EFFECT DURING ELECTROLYSIS WHEN PRESENT IN TOO HIGH A CONCENTRATION, WHICH COMPRISES NEUTRALIZING THE SOLUTION BY THE ADDITION THERETO OF LIME AND MAGNESIA IN ADDITION TO ANY LIME OR MAGNESIA ORIGINALLY CONTAINED IN THE 